The present project is a phase of an ongoing investigation designed to elucidate mechanisms controlling growth rate. The work consists of studies of experimental growth retardation and catch-up (compensatory) growth in the Long-Evans rat. In this study growth retardation is produced by fasting, cortisone injections, or propylthiouracil feeding. During periods of growth retardation and recovery we will investigate 1) patterns of pulsatile release of growth hormone during 24 hour periods, 2) running activity and energy intake cycles in blinded rats during the treatment and recovery periods, 3) energy intake and losses in urine and feces in other non-blinded treated and recovering animals, 4) properties of a factor in normal rat serum which inhibits thymidine uptake in vitro by chondrocytes, and 5) the effects of parabiosis during the recovery phase of control rats to treated rats on cartilage metabolism.